It has been possible for the last year or so to construct fine structure maps of selected eukaryotic gene systems down to the nucleotide level via cloning and DNA sequencing. The number of cloned eukaryotic gene systems is growing rapidly. We would like to explore the function of eukaryotic nucleotide sequences by the classical approach of studying the behavior of mutant sequences. Two gene families are being collected to provide sequence variants; a small 5S gene family (3-4 species) and a large family of mouse globin genes (15-30 variants). The members of each family will be compared in detail by restriction enzyme analyses, heteroduplexing and sequence analyses. This comparison will serve as a starting point to generate models which will be tested by in vitro mutagenesis of the globin sequences. Variants will be generated by site specific and site directed mutagenesis. Assay systems in which the in vitro altered sequences will be tested for function will be the putative thymidine kinase vector and the Xenopus transcription systems. The genetics of mouse globins will as a consequence be explored in detail at the DNA level.